The mats that we use are expensive, unless you can get them at trade prices. But before you ever get the mat you need to line the shelf with polystyrene sheeting, then metal foil, next is the heating mat, another layer of foil, capillery matting comes next. and lastly a perforated plastic sheet. A sensor comes out of the heat mat which you push into the compost in one of the seed trays you are using. You can then set the thermostat to the desired temperature, I run between 20 and 25c. The mister is an automatic watering system with all the watering nozzles changed to very fine bore jets. which comes on evening time for 10 minutes which is enough to water the plants and soak the capillery matting with enough water for the next day. I hand spray once a week with a cheshunt mixture to stop mould and damping off. These are also ideal conditions for whitefly, so a couple of those big sticky sheets hung up keep them down. Mousetraps set every night just in case a little bugger gets in, as they will nibble the cables and cost you a fortune.
